The continuing push to produce faster semiconductor devices with lower power consumption has resulted in device miniaturization. In particular, smaller gate oxide thickness and silicon channel width are conducive to the low voltage and faster operation of transistor devices, such as complementary metal oxide (CMOS) transistors. With shrinking process geometries, the use of new materials is being explored to further reduce power consumption and increase device switching speeds.
In an N-type metal Oxide Semiconductor (NMOS) transistor, for instance, a channel made from a silicon layer that is epitaxially grown on a silicon-germanium substrate has an increased electron mobility. This, in turn, allows the production of NMOS transistors having faster transistor switching speed and higher drive current.
Increased electron mobility is thought to be due to the presence of biaxial tensile strain in the NMOS channel. It is known that the wider lattice spacing of the silicon-germanium substrate causes the lattice spacing of silicon atoms in the silicon layer to be stretched or strained to match that of the silicon-germanium substrate. Strain in the channel occurs biaxially, that is, in directions parallel and perpendicular to the flow of current through the channel.
In contrast, the use of biaxially tensile-strained-silicon in a P-type Metal Oxide Semiconductor (PMOS) transistor is much less beneficial. A PMOS channel formed in biaxially tensile strained silicon has little, if any, improvement in hole mobility compared to an equivalent channel formed in unstrained silicon. This is a major barrier to preparing CMOS semiconductor devices on strained silicon layers, where both PMOS and NMOS transistors are present. This follows because the drive current of both types of transistors must be increased to realize an improvement in device performance. One approach to increase the drive current of PMOS transistors is to form epitaxial silicon-germanium source/drain structures that provide uniaxial compressive stress to the channel in a direction parallel to the electron flow.
Accordingly, what is needed in the art is an improved method of manufacturing MOS transistors on strained-silicon that improves the drive current for both NMOS and PMOS transistors while not suffering the deficiencies of previous approaches.